1. Field of the Invention
The invention concerns an exhaust gas turbocharger for an internal combustion engine, that is, an exhaust gas turbocharger for an internal combustion engine, with at least one turbine in the exhaust gas flow downstream of the internal combustion engine, wherein the turbine includes at least one flow channel with a radial flow-inlet cross-section, with a flow ring or nozzle ring bordering the radial flow-inlet cross-section, and wherein a variable guide vane arrangement is provided in this area for varying the radial cross-section.
2. Description of the Related Art
One exhaust gas turbocharger of this type is described for example in German Patent DE 100 29 640 C2. Such an exhaust gas turbocharger will be referred to in the following as a turbocharger with variable turbine geometry (VTG-turbocharger). Additional turbochargers with variable turbine geometry are described in DE 100 48 105 A1, DE 43 30 487 C1 as well as DE 196 15 237 C2.
One VTG-turbocharger known for example from DE 100 29 640 C2 has both a radial flow-inlet cross-section and a semi-axial flow-inlet cross-section, which flow into the turbine segment containing the turbine wheel along a radial or, as the case may be, semi-axial flow entry section. In the flow entry section a flow-optimizing continuous flow ring is provided, which borders and defines the flow entry cross-section in appropriate manner. In the radial flow entry section a guide ring with variable guide vanes is provided, via which the radial flow entry cross-section is varied as desired, so that a variable turbine geometry is provided. Depending upon the actual operating condition of the internal combustion engine, these guide vanes are adjustable between a position restricting free flow of the entry section towards the turbine wheel and a position of wide open flow entry cross-section, whereby the exhaust gas back pressure in the pipe segment between the cylinder outlet and the exhaust gas turbine is manipulated and thus can be adjusted to a desired value. By the adjustment of the guide vanes, it becomes possible to influence the exhaust gas back pressure as well as the mode and manner of the flow of the exhaust gas onto the turbine wheel, whereby the output of the turbine and therewith the output of the compressor can be adjusted as desired depending upon the operating condition of the internal combustion engine. This type of exhaust gas turbine can be used both in the propulsion mode of the internal combustion engine for increasing the motor output as well as in a motor braking operation for providing supplemental motor braking power.
During motor braking the guide vanes are brought to a flow blocking position in order to achieve a high pressure level both on the exhaust gas side as well as on the charge air side for a desired high motor brake power. This occurs by significantly reducing the entry cross-section. The desired high braking power can however only then be achieved, when within the desired pressure distribution exists in the turbine and the exhaust gas flows through the turbine in a pre-determined and precisely controllable manner.
The problem therewith is, in particular, the undesired air bypass (flow-by) between the flow-inlet channel of the turbine and the exhaust side of the turbine, which bypass can occur due to component and manufacturing tolerances, and also due to wear and thermal expansion within the turbine. These undesired bypass air flows occur for example at the large gaps, which can form in the area of the end faces of the radial guide vanes at or opposite their pivotable mounting as a result of thermal or mechanical loads. These bypass flows can strongly interfere with the desired pressure distribution within the turbine, which can have negative consequences both on the motor output in the combustion drive mode however as well as on motor braking power. Bypass flows can occur however also in the gap which is provided during construction as required for the moveability of the guide vanes in one of the flow entry cross-sections.
The problem of undesired flow bypass is of high importance in particular in commercial vehicle motors, which must produce a high motor brake power. There, in particular to avoid bypass flows, a particularly precise fitting mounting of the guide vanes and at the same time reduced thermal and mechanical loading is desired.
In German Patent document DE 100 29 640 C2 an exhaust gas turbocharger of this type is described, in which the positions of the flow ring in the housing of the turbocharger can be variably adjusted. The flow ring, which has the function of limiting the radial or as the case may be the semi-axial flow entry cross-section to a desired value, is here designed to be axially slideable, whereby the otherwise existing guide vane gap in the radial grid may be reduced to a minimal amount.
A problem with the solution described in DE 100 29 640 C2 is that for avoidance or reduction of exhaust flow bypass the entire guide vane ring must be displaced. In particular in commercial vehicles, in which a very high motor brake power is necessary therewith also the motors exhibit a corresponding large size, this flow ring for limiting the radial and semi-axial flow entry cross-section is relatively large. The gap between the variable guide vanes and the flow ring or nozzle ring on the other hand should be as small as possible, ideally in the range of a few tenths to hundredths of millimeters. In practice there exists therewith the problem, of adjusting the relatively large and therewith also massive flow ring in the axial direction defined to a few tenths or hundredths of millimeters of precision. Such a precise adjustability is not possible in practice, or only with substantial complexity. For this in particular a very complex adjustment ring must be provided, with which the complexity is frequently not justified by the use as a supplemental motor brake device.
One such arrangement functions in satisfactorily manner only when no large temperature fluctuations occur. Turbochargers are however subjected to very strong temperature fluctuations due to the flow through of hot exhaust gases, in particular in the case of high power or performance, so that the turbine parts and adjacent parts can heat up to 900° C. These frequent and high temperature oscillations together with the extreme high RPM of the turbine wheel in the compressor wheel produce extreme loads and stresses for all components of the exhaust gas turbocharger, which result in a short life and a loss of function of the turbocharger.
In particular in the case of large motor capacities, including with the arrangement known from DE 100 29 640 C2, a compromise must be made between compensating for a relatively large gap and providing a small as possible gap between guide vanes and nozzle ring. In this case however the employment of an axially displaceable nozzle ring would no longer be justified. In other cases however the nozzle ring as well as the end faces of the guide vanes of the guide arrangement would be subjected to such a substantial frictional wear, that the life of the turbine wheel and therewith the economic feasibility would be significantly reduced. It is however highly desirable to avoid as much as possible reducing the useful life, particularly in the case of turbochargers.